A property often demanded for microscopes and binocular magnifiers is that the working distance of the optical device be capable of modification within a specific range, typically between 200 mm and 400 mm (also called the “focusing range”). At present, this is achieved by the fact that at least one optical element (for example, a lens element or a lens group) is shifted axially in the optical axis, bringing about a modification in focal length. This usually results in a change in overall length, cf. “Sigma Lens Catalog,” Tokyo 201, 3 pages (no publication data), obtained in year 2000.
A presbyopic eye (having a lens no longer capable of accommodation) sees clearly only at one specific distance. This disadvantage is overcome by the use of eyeglasses. Lens elements in the form of a progressive lens have also recently become available. In a progressive lens, the refractive power of the corrective lens changes laterally (non-axially) over the surface of the eyeglass lens. As the eye looks through the areas of differing refractive power, for example from top to bottom, it can then see clearly at different distances despite poor accommodation.
Progressive lenses used today are made of inorganic optical glass materials or of plastics, e.g. polycarbonate, etc.
It is recognized that focusing on an object by means of axially displaceable lens elements or lens groups, as in the case of conventional microscopes, binocular magnifiers, etc., is disadvantageous in terms of the following aspects:                a) Changes in overall length.        b) Greater weight because of greater overall length.        c) Complex design solutions in some cases, if focusing is to occur in “open space,” i.e., without a fixed housing having the corresponding mechanisms, for example in a binocular magnifier.        d) Lens groups are difficult to exchange, and the range of application for binocular magnifiers or microscopes is therefore limited.        